How to submit a client's MTD VAT using thier own HMRC login details when QB is connected through our HMRC agent service account.

Hello Info1533, 

Welcome to the Community page, 

So what you have to do is log into your clients account as a company admin and not as the accountant  and then use the clients information to then submit the vat for the client. You might need to go through the authorisation process as the Company admin user if it hasn't already been done so you can then submit the vat on behalf of the client using their details instead of using your accountant details though agent tools.